This invention concerns a process to produce metallic iron starting from mineral iron, wherein the iron is present in the form of oxides, and the relative apparatus which comprises a reduction furnace which may have one or more inlets for the reducing gas and inside which the process of direct reduction of the iron (DRI) is carried out. The reducing gas is obtained by mixing a part of the process gas, which emerges from the reduction furnace, with additional gas arriving from an outside reforming circuit.
The state of the art includes processes of direct reduction which use the injection of hydrocarbons into the current of reducing gas to allow the reaction of reforming the methane in the furnace with the H2O and CO2 in the gas; there are also known processes of direct reduction which use the injection of hydrocarbons with C greater than 5 directly into the furnace in the zone between the injection of the reducing gas and the outlet from above of the burnt gas.
From the following patent documents other processes are known for the direct reduction of mineral iron:
The state of the art also includes processes wherein the hot metallic iron is produced in a reduction furnace of the shaft type, with a vertical and gravitational flow of the material, which is subsequently sent to the melting furnace by means of a closed pneumatic transport system in an inert atmosphere.
The method according to the invention consists in bringing into contact the mineral iron, of various granulometry, with a feed gas in a reduction furnace of the shaft type, wherein both the gas and the material are fed continuously, so that a vertical and gravitational flow of material is created and the direct reduction of the mineral is achieved. The material may be discharged from the reactor either cold or preferably hot to be sent subsequently to a melting furnace or so that it may be converted into hot briquette iron (HBI) or cooled and converted into direct reduction iron (DRI).
The reduction furnace is equipped with means to feed the mineral iron and means to discharge the reduced metallic iron; it is equipped with at least one inlet collector to inject the reducing gas in correspondence with a reduction zone or reactor inside the furnace.
The reducing gas sent into the reactor contains hydrocarbons injected into the current after the partial combustion of the hydrogen and carbon monoxide with the oxygen and is obtained by mixing a part of the process gas, which exits from the reduction furnace, with additional gas arriving from an outside reforming circuit.
In a variant, the hydrocarbons are injected before the partial combustion is achieved, with the purpose of raising the temperature of the gas introduced into the reactor.
According to another variant, the hydrocarbons are at least partly injected into a zone between the reduction zone and the zone where the reduced material is discharged.
In all cases, the injected hydrocarbons cooperate in reducing the iron oxide (FeO) to metallic iron, generating more H2 and CO.
The direct reduction of the iron oxides is achieved in two different continuous stages inside the reduction reactor.
In a particular embodiment, the furnace is provided with a first stage, defined as the pre-heating and pre-reduction stage, where the fresh iron oxides, that is, those just introduced into the furnace, come into contact with a mixture of reducing gas, consisting of partly burnt gas, arriving from the underlying part of the furnace and of fresh hot gas, that is, gas introduced from outside, arriving from a collector which brings fresh reducing gas and possibly CH4 or other natural gas. This first stage takes place in a corresponding first zone arranged in the upper part of the furnace.
In the second stage, the reduction stage proper, the complete reduction of the iron oxides is achieved, due to the action on the oxides, already partly reduced in the first stage, of a mixture of reducing gas based on H2 and CO and at least a hydrocarbon, preferably natural gas, injected in the median zone of the reduction reactor. This second stage takes place in a corresponding second zone arranged below the first zone.
The two inlets to the furnace through which the gas is introduced can be independently regulated both in the flow of fresh reducing gas and in the addition of natural gas in the current introduced.
Moreover, the inlet temperature of the two currents of reducing gas can be independently regulated by injecting O2 before they enter the reduction reactor.
The oxidation reaction needed to raise the temperature of the gas leads to a change in the level of oxidation of the gas, from normal values of 0.04-0.08 to 0.06-0.15.
The following ratio is intended for the level of oxidation of the reducing gas:
Nox=(H2O+CO2)/(H2O+CO2+H2+CO)
In the second reaction zone of the furnace, wherein the reduction of the iron oxides is completed, a gas is generated with a high content of H2 and CO and with an oxidation level of between 0.15 and 0.25 due to the reduction reactions of the iron oxides with H2, CO and CH4.
Once this gas has left the second reaction zone, it enters the first reaction zone, located higher up, and mixes with the hot gas injected into the first zone to pre-heat and pre-reduce the iron oxides.
The gas emerging from the reduction reactor is partly recircled and partly used as fuel.
The recircled gas has a volume composition within the following fields:
According to one characteristic of the invention, the gas feeding the reduction reactor consists of a mixture of natural gas, recircled gas, also known as process gas or top gas, which exits from the reactor itself, and reformed gas; the recircled gas is pre-heated to a temperature of between 650xc2x0 C. and 950xc2x0 C.; the gas emerging from the pre-heater is in turn mixed with fresh reformed gas and subsequently with air, or air enriched with oxygen, or pure oxygen, to carry out a partial combustion of the H2 and CO in the reducing gas in order to raise the temperature to values of between 800xc2x0 C. and 1150xc2x0 C., preferably between 1000xc2x0 C. and 1150xc2x0 C.; and the oxidation level of the resulting gas feeding the furnace is between 0.06 and 0.15.
The methane represents between 6 and 20% in volume of the mixture of reducing gas.
When the feed gas comes into contact in the reduction zone with the hot, partly reduced material, which therefore consists partly of metallic iron and partly of iron oxides, a highly endothermic reaction is produced.
There is a also an endothermic reaction in the pre-heating and pre-reducing zone when the gas comes into contact with the iron oxide.
One advantage of this invention is that the first pre-heating and pre-reducing zone is extended, which allows to start the transformation of the Ematite (Fe2O3) into Wustite (FeO) more quickly.
The whole reactor works at a higher average temperature and above all which is constant along both zones, both the pre-reduction and reduction zones, encouraging a higher reaction speed, with a consequent effect of reducing consumption and increasing productivity.
In the case where the furnace has two inlets for the introduction of the reducing gas, the first inlet is located at a set distance (x) with respect to the second inlet, which is located in the median part of the furnace, in correspondence with the second reduction zone. This distance (x) is suitably between 1 and 6 meters, preferably between 2 and 4 meters, to encourage the reactions in the most suitable zone between the reducing gas and the iron oxides.
The first gas inlet also has the function of pushing the gases arriving from the second reduction zone towards the center of the furnace so as to create a uniform distribution of the gas in the section of the reactor.
According to a variant, there are multiple, or more than two, inlets for the reducing gas into the furnace. The first current of reducing gas is injected into the middle of the reactor, into the reduction zone proper, while the other currents are introduced into the zone between the injection of the first current of gas and the outlet of the burnt gas, in the upper part of the furnace. This intermediate zone will be called the pre-heating and pre-reducing zone for the iron oxide based material.
The flow of gas into the reactor thus composed allows to have the whole reduction and pre-reduction zone at as constant a temperature as possible, and to have a gas inside the furnace which always has a high reducing power, encouraging a greater productivity and a lower consumption of gas; this also allows to improve the final metalisation of the product.
In this way, moreover, the iron oxides arrive at the reduction zone already partly reduced, thus encouraging the completion of the final reduction reaction from FeO to Fe.